Police have charged two more suspects in a spate of deadly shootings at the Indonesian gold mine of US company Freeport, but the motive for the attacks remains a mystery, officials said Friday.

The latest arrests take to nine, the number of people facing charges of premeditated murder and illegal weapons possession, for the series of ambushes that left three dead earlier this month at the Grasberg complex in Papua, the largest gold mine in the world.

Among those charged are two Freeport employees, but their role is still unclear, AP reported on Friday.

A 29-year-old Australian worker and a Freeport guard suffered fatal gunshots, while a police officer fell to his death seeking cover from gunfire. It is the worst violence at the site since three schoolteachers, two of them Americans, were killed in 2002.

Papua Police chief Bagus Ekodanto said Friday it was still unclear if the culprits were members of the Free Papua Movement, which has waged a low-level insurgency against the government for 40 years, or a different armed group, according to AP.

Freeport has been targeted with arson, roadside bombs and blockades since production began in the 1970s during the US-backed Soeharto dictatorship.

It is also regularly the focus of protests by local residents who feel they are not benefiting from the depletion of Papua's natural resources.

Minister of Defense Juwono Sudarsono said recently Freeport should not be shut down because of the security problems as "Freeport gives a lot to state revenue".

The Indonesian Forum on the Environment (Walhi) disagreed, with executive director Berry Nahdian Forqan saying it was high time for the government to act firmly against the mining giant.

"We urge the government to halt Freeport's operations in the country because of the ecological, social and economic damage it has caused Papuans over the decades," he said.